Rotated Digits

X is a good number if after rotating each digit individually by 180 degrees, we get a valid number that is different from X. Each digit must be rotated - we cannot choose to leave it alone.

A number is valid if each digit remains a digit after rotation. 0, 1, and 8 rotate to themselves; 2 and 5 rotate to each other (on this case they are rotated in a different direction, in other words, 2 or 5 gets mirrored); 6 and 9 rotate to each other, and the rest of the numbers do not rotate to any other number and become invalid.

Now given a positive number N, how many numbers X from 1 to N are good?

Example:

Input: 10

Output: 4

Explanation: 

There are four good numbers in the range [1, 10] : 2, 5, 6, 9.

Note that 1 and 10 are not good numbers, since they remain unchanged after rotating.

Note:

N will be in range [1, 10000].

Problems:

using System;
using System.Collections.Generic;
using System.Text;

namespace LeetCode.AskGif.Easy.String
{
 class RotatedDigitsSoln
 {
 public void execute()
 {
 var res = RotatedDigits(10);
 }

 public int RotatedDigits(int N)
 {
 int ans = 0;
 for (int i = 0; i <= N; i++)
 {
 if (IsRotatedValidNumber(i)) ans++;
 }
 return ans;
 }

 private bool IsRotatedValidNumber(int i)
 {
 bool flag = false;
 while (i != 0)
 {
 var remainder = i % 10;
 if (remainder == 3 || remainder == 4 || remainder == 7) return false;
 if (remainder == 2 || remainder == 5 || remainder == 6 || remainder == 9)
 flag = true;
 i = i / 10;
 }
 return flag;
 }
 }
}

Time complexity: O(n^2) ( it will be O(n) if the length of the digit is finite)

Space complexity: O(1)